The invention relates to a method for adapting the radiation dose of an X-ray source which irradiates an object to be examined so as to form an X-ray image of the object. The invention also relates to an X-ray device for carrying out such a method.
In order to form X-ray images of an object to be examined, for example, a workpiece in the case of industrial applications or the body of a patient in the case of medical applications, the object is irradiated by means of X-rays. The transmitted X-rays are detected by an imaging detector so as to be converted into an image of the absorption density distribution. A situation is often encountered in which only a part of the fluoroscopy image is of interest for the purpose of the examination. The radiation dose of the X-ray source, therefore, should be adjusted in such a manner that such a region of interest is optimally imaged. In the case of medical applications, image regions which are not of interest occur notably when X-rays bypass the body and are incident directly on the detector (direct radiation) and hence do not contain any information concerning the absorption by the tissue. Furthermore, image regions which do not receive X-rays because of the filtering effect of absorption filters are not of interest either.
In order to adapt the X-ray dose to the X-ray image it is known to measure the overall dose arriving in the overall measuring field of the X-ray detector. When direct radiation occurs in a given, typically small part of the measuring field, a high overall dose is measured; consequently, the X-ray dose is controlled to a value which is smaller than necessary. In order to avoid this effect, it is known from WO 98/48600 to determine the histogram of the grey value distribution of the detected fluoroscopy image and to define therefrom a grey value threshold in conformity with given criteria which may be defined, for example, by fuzzy logic rules. All image points having a grey value below the defined threshold then per definition belong to the region of interest. The adaptation of the X-ray dose is subsequently performed while taking into account only the region of interest, that is, the points whose grey value is below said threshold value. This method has the drawback that exclusively the grey value of an image point decides whether or not this image point belongs to the region of interest. However, notably isolated points with deviating grey values could thus be unduly assigned to the region of interest.
Considering the foregoing it is an object of the invention to provide a method and a device for the adaptation of a radiation dose of an X-ray source which enable more correct adaptation to the regions of interest.
The method is intended to adapt the radiation dose of an X-ray source which irradiates an object to be examined, for example, the body of a patient, thus producing an X-ray image of the object. According to the method the X-ray image is subdivided into coherent image regions which have a predetermined minimum format and the radiation dose is adapted so as to be optimum for the image regions of interest. The image regions of interest are defined in that their mean grey value satisfies a given criterion.
According to the described approach the region of interest is not defined one image point or pixel after the other, but is composed of image regions of a predetermined minimum format, that is, regions comprising a minimum number of pixels. This approach ensures that the image region of interest consists of coherent parts and that no isolated islands of only one or a few pixels are included in the image region or that, conversely, no small holes of only one or a few pixels exist in the image region. The method thus produces a more realistic definition of the image region of interest, so that the subsequent adaptation of the X-ray dose on the basis of this image region of interest offers a better result. Furthermore, it is advantageous that the evaluation of image regions of a given minimum format usually can be performed faster than the evaluation of all pixels individually. The minimum format of the image regions is typically from approximately 100 to 100,000 pixels.
There are various possibilities for subdividing the X-ray image into coherent image regions of a predetermined minimum format. In the simplest case the X-ray image is subdivided into a regular grid of rectangular image regions, all of which may have the same format. Because in many cases the location in which a boundary of the image region of interest is most likely to occur is known, the format of the image regions may also be chosen so as to be smaller in such a boundary region, thus achieving a better resolution as regards the course of the boundary.
In a preferred version of the method all coherent points of the locally low-pass filtered X-ray image whose grey values lie in a given interval are combined so as to form an image region. Thexe2x80x9ccoherencexe2x80x9d of the points means that the image region forms a geometrically coherent surface, so that each pair of points of the image region can be interconnected by a line extending in the image region. The low-pass filtering of the X-ray image ensures that large brightness gradients are compensated or xe2x80x9cspread outxe2x80x9d so that isolated pixels of deviating brightness are avoided.
There are various possibilities for defining the given criterion which must be satisfied by the mean grey value of an image region in order to allow the image region to be an xe2x80x9cimage region of interestxe2x80x9d. In conformity with a first version this criterion consists in that the mean grey value of the corresponding image region must be larger than a minimum value and/or smaller than a maximum value. The minimum value and the maximum value may then be fixed or be defined in dependence on the relevant situation. For example, the values may be based on the mean grey value of the overall X-ray image. It is notably possible to specify only a maximum value which corresponds to a given percentage of, for example, from 100% to 200% of the mean grey value of the overall X-ray image. Image regions having a mean grey value exceeding this maximum value are then no longer considered to form part of the image region of interest. Notably image regions which receive direct radiation can thus be excluded. The exclusion of image regions whose mean grey value is below a minimum value, however, enables the exclusion of image regions from the image region of interest which correspond to zones masked by absorption filters.
According to a further version of said criterion the image regions which are (potentially) of interest are iteratively determined on the basis of the total number of image regions in that image regions which are not of interest are successively separated in conformity with the following steps:
a) separating the brightest image region from the total number of potential image regions of interest,
b) determining the mean grey value of the image region separated in the step a),
c) determining the mean grey value of all potential image regions of interest remaining after the separation in the step a),
d) starting the iteration again with the step a) if the pair of mean grey values determined in the steps b) and c) lies outside a predetermined characteristic number of value pairs, the previously separated image region no longer being included in the number of potential image regions of interest; however, if the pair of grey values determined in the steps b) and c) lies within said characteristic number, the iteration is terminated and the number of image regions of interest is identified as the number of potential image regions of interest prior to the last execution of the step a).
The described iterative approach enables a simple determination of the image regions of interest while excluding image regions which have a very high mean grey value, for example, because of direct radiation. The dose of the X-ray source can thus be adapted also in the presence of direct radiation, without detailed knowledge of the system parameters being required for dose control.
The characteristic number used in the step d) of the above iterative method so as to make a distinction between image regions which are of interest and those which are not, can be defined in various ways, notably experimentally. Preferably this characteristic number is defined in such a manner that the pair of the mean grey values determined in the steps b) and c) lies outside this characteristic number when the ratio of the mean grey value of the separated image region to the mean grey value of the remaining, potential image regions of interest exceeds a threshold value. This threshold value may typically lie in the range of from 1.5:1 to 10:1.
The invention also relates to an X-ray device which includes the following elements:
an X-ray source for emitting X-rays,
an X-ray detector for forming an X-ray image of an object irradiated by the X-ray source,
a control unit for adapting the radiation dose of the X-ray source, the control unit being arranged in such a manner that it is capable of carrying out a method of the kind set forth.
An X-ray device of this kind provides a realistic definition of an image region of interest which is taken into account for the adaptation of the radiation dose of the X-ray source. The X-ray device is thus capable of producing X-ray images of higher quality of objects to be examined.